And if that makes for a merry Christmas for viewers, there's even more to come in the New Year as Doctor Who celebrates its 50th anniversary: "They are immense, they are considerable and they will be full of tremendous surprises," said writer Steven Moffat of his much-discussed anniversary plans.

"We're not going to tell you [any details] but I promise we're going to take over television, trust me," he told an audience of fans at a special preview of the Christmas episode.

Certainly Matt Smith will be around to play the titular Time Lord – he intends to go on playing it "for infinity number of years" he said.

It is not as if the 60-minute Christmas special – which perhaps controversially comes sprinkled with a seasonal splash of romance – is unambitious: among the co-stars is Richard E Grant as the deliciously wicked Dr Simeon, a man dreaming of a white Christmas.

Set in Victorian London, with the Doctor still grieving the loss of Amy and Rory following his final adventure with the Ponds, we meet Clara (Jenna Louise Coleman), the doctor's new companion, as she finds herself in the middle of a plan that could be very bad news.

Regular viewers have already met Coleman before, of course, after she made a surprise appearance as Oswin in Asylum of the Daleks – the first show of the last series. Even more confusing than the name change is that the last time we saw her she died.

Joining as the new companion was an interesting experience she said: "I felt like I was on a blind date for a while … it's like putting two people together, 'Oh I know someone you'll really get on with – now go on and save the world'."

But the Doctor doesn't just have Clara for company in next week's episode, he is also joined by friends from series past – including the Silurian Madame Vastra and her partner Jenny, and Sontaran Commander Strax. And other Steven Moffat creations try to bust on to the screen to often hilarious effect, as the showrunner sends up his own trademark writing style.

Those hoping for a glimpse of Benedict Cumberbatch as Sherlock, however, will be disappointed. He will never share a screen with the Timelord, said Moffat.

"The doctor would just be furious because there would be someone else clever and tall in the room, and that's his job," said Steven Moffat. "Can you imagine it? They'd just stand there and sulk at each other."

The Doctor might not have Sherlock's covetable greatcoat,but he does have a new look for the new series: stepping out as a Victorian gent in waistcoat, plaid trousers and top hat. "It's quite a collaborative experience," said Smith. "I've been hankering after something purple for a while."

Also getting a new look is the Tardis, which has had an appealingly minimalist makeover. "I thought we'd been getting progressively whimsical with the interior of the Tardis," explained Moffat.